The present invention relates to electrical discharge machining processes and apparatus for cylindrical works which are designed to satinize by electrical discharge the outer surface of a cylindrical work to a desired surface roughness, and more particularly the invention relates to an electrical discharge machining process and apparatus capable of producing a satin-like roughened surface of a predetermined roughness on the outer surface at each end of a cylindrical work in the same manner as the remaining outer surface of the work.
In the past, the method of throwing hard metal particles, e.g., shot or grit, at the polished surface of strip rolling rolls, particularly cold rolling rolls, to produce impressions on the roll surface, has been used as a means of giving a satin-like finish to the surface of rolls, and recently attempts have been made to replace this type of process by electrical discharge machining. As is well known, electrical discharge machining is a process in which an insulating liquid such as kerosene is placed in a small discharge gap between an electrode and a work piece and a pulse voltage is periodically applied across the electrode and the workpiece to cause a discharge and thereby to work the surface of the workpiece. By repeatedly performing this electric discharge machining at the surface of a roll while rotating the roll in the circumferential direction and also gradually moving the electrode along the axis of rotation of the roll, it is possible to continuously give a spiral satin finish to the roll surface and thereby to produce spark eroded impressions all over the roll surface. This is the known method of giving a uniform satin finish to the surface of a roll by means of electrical discharge machining. The satinized surface produced in this way has many advantages, that is, not only the roughness is more marked and the shape is in excellent trim as compared with the mechanical impressions produced by the throwing of metal particles, but also the shape is not affected by the manufacturing method, hardness, etc., of the roll, and the metal structure in the roll surface is hardened by the electrical discharge thus making the roll optimum for rolling purpose and so on.
However, a disadvantage of this process relates to the working of the roll ends. More specifically, while the working of the roll surface by means of shot blasting or the like is capable of giving a uniform satiny finish all over the roll surface including the roll ends, the machining process by electric discharge is not capable of uniformly working the ends of the roll.
This deficiency will now be described in greater detail with reference to FIG. 1 of the accompanying drawings which shows a prior art electric discharge machine. In the figure, numeral 10 designates a working pulse source unit, 12 an electrode made for example of copper, 14 a roll to be subjected to satinization, 16 a holder of the electorde 12, 18 a head column for controlling the working gap between the electrode 12 and the roll 14, 20 a holder for the head column 18, 22 a sliding shaft for sliding the holder 20 in the direction of the arrow shown in the Figure. (in the lengthwise direction of the roll axis), and 24 a feed screw for the holder 20 which is adapted for computing the working area of the electrode 12. Numeral 28 designates a first reduction gear, 30 a feed screw rotating motor for sliding the electrode 12 along the sliding shaft 22, 32 roll supports for rotatably supporting the roll 14, 34 a second reduction gear, 36 a roll rotating motor, and 38 a feeding brush unit for feeding power to the roll 14.
With the machine described above, the electric discharge machining process will now be described. In the working of the roll 14, working conditions are first predetermined for the working pulse generating unit 10. The working conditions determine the surface roughness of a satin finish to be given to the surface of the roll 14. The working gap between the electrode 12 and the roll 14 is immersed in an insulating liquid such as kerosene and a pulse voltage is applied from the pulse source unit 10, thus causing a discharge and thereby performing electrical discharge machining. In this case, simultaneously the roll 14 is rotated and the electrode 12 is gradually moved along the axis of rotation of the roll 14, for example, from the left to the right in the Figure.
In other words, the electrode 12 works the surface of the roll 14 while spirally moving along the surface of the roll 14. The speed of feeding the electrode 12 along the roll axis is for example selected so that the desired satiny finish is given to the entire surface of the roll 14 for every movement of the electrode 12 past the roll 14. When the desired surface roughness for working is 18.mu. Rmax and the roll diameter is on the order of 600.phi., then the proper feed speed will be on the order of 2.5 mm/min. The rotational speed of the roll 14 should not be excessively low. Although the entire surface can still be machined by rotating the roll 14 so that the electrode 12 moves by an amount which is little less than the electrode width for every rotation of the roll, this tends to cause spiral scores on the surface of the roll 14 which are undesirable for rolling rolls, although practically no difference is perceivable by visual comparison. As a result, it is desirable that the rotational speed of the roll is as high as possible so long as the electrical discharge machining can be effected stably.
In other words, the working rate per roll rotation decreases with an increase in the rotational speed of the roll 14, thus eliminating the possibility of causing spiral scores on the surface of the roll 14. In this way, a uniform satin-finished surface is successively produced on the entire surface of the roll 14. However, this process cannot be successibly applied to the working of the ends of the roll 14. Namely, where the operation is terminated in response to the arrival of the electrode 12 at the end of the roll 14, if the working conditions have been preset to satisfy the minimum requirement of producing a satin-finished surface on the roll surface during the working of the central portion of the roll surface, this tends to leave at the end an unworked portion of a width corresponding to the width of the electrode 12. On the other hand, if the feed speed of the electrode 12 is decreased only during the periods of working the roll ends, the working rate at each end of the roll 14 will be increased thus failing to ensure uniform working of the entire surface. Still further, when the electrode 12 reaches the end of the roll 14, if the operation is not stopped and it is instead continued until the electrode 12 moves past the roll 14, the electrode 12 moves past the end of the roll 14 and the surface area worked is reduced with the resulting increase in the working rate per unit area, thus, as compared with the case where the entire face of the electrode 12 faces the roll 14, gradually increasing the depth of spark erosion and thereby forming a spiral stepped portion on the surface of the roll 14.